U.S. patent number 6,190,471 [Application Number 09/318,635] was granted by the patent office on 2001-02-20 for fabrication of superalloy articles having hafnium- or zirconium-enriched protective layer.
This patent grant is currently assigned to General Electric Company. Invention is credited to Ramgopal Darolia, William S. Walston.
United States Patent |
6,190,471 |
Darolia , et al. |
February 20, 2001 |
Fabrication of superalloy articles having hafnium- or
zirconium-enriched protective layer
Abstract
A superalloy article has a protective layer thereon, either in
the form of an environmental coating or a the bond coat for a
thermal barrier coating system. The protective layer has a high
content of hafnium and/or zirconium to improve the adherence and
properties of the protective layer. To introduce the hafnium and/or
zirconium into the protective layer, the nickel-base alloy
substrate, to which the protective layer is applied, is prepared
with an initially elevated content of the hafnium and/or zirconium.
A conventional bond coat is applied to the substrate. In an
interdiffusion treatment performed during coating and/or
subsequently, hafnium and/or zirconium diffuses from the substrate
into the bond coat.
Inventors: |
Darolia; Ramgopal (West
Chester, OH), Walston; William S. (Maineville, OH) |
Assignee: |
General Electric Company
(Cincinnati, OH)
|
Family
ID: |
23238985 |
Appl.
No.: |
09/318,635 |
Filed: |
May 26, 1999 |
Current U.S.
Class: |
148/537; 148/527;
428/680 |
Current CPC
Class: |
C23C
28/345 (20130101); C23C 28/321 (20130101); C23C
28/3455 (20130101); C23C 28/3215 (20130101); C22C
19/057 (20130101); C23C 28/325 (20130101); C23C
26/00 (20130101); Y02T 50/60 (20130101); Y10T
428/12944 (20150115) |
Current International
Class: |
C23C
28/00 (20060101); C23C 26/00 (20060101); C22C
19/05 (20060101); C22F 001/00 (); C23C 004/00 ();
C23C 010/00 () |
Field of
Search: |
;148/527,537
;428/680 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jenkins; Daniel J.
Assistant Examiner: Coy; Nicole
Attorney, Agent or Firm: Hess; Andrew C. Narciso; David
L.
Claims
What is claimed is:
1. A method for preparing a single-crystal superalloy article
having a protective layer thereon, comprising the steps of
selecting a nickel-base superalloy composition;
preparing a modified version of said nickel-base superalloy
composition, wherein the modified version of said nickel-base
superalloy composition has an excess of a protective-layer
modifying element over that of the nickel-base superalloy
composition, the protective-layer modifying element being selected
from the group consisting of
from about 0.2 to about 2.0 percent by weight hafnium, and
from about 0.1 to about 0.5 percent by weight zirconium, and
combinations thereof;
processing the modified version of said nickel-base superalloy
composition into a substrate having the shape of the article and
being substantially a single crystal;
applying a protective layer to a surface of the substrate, the
as-applied protective layer having a lower concentration of the
protective-layer modifying element than the substrate; and
interdiffusing the protective layer modifying element from the
substrate into the applied protective layer.
2. The method of claim 1, including an additional step, after the
step of applying a protective layer, of depositing a ceramic layer
overlying the protective layer.
3. The method of claim 1, wherein the protective-layer modifying
element is hafnium.
4. The method of claim 1, wherein the protective-layer modifying
element is zirconium.
5. The method of claim 1, wherein the protective-layer modifying
element is a combination of hafnium and zirconium.
6. The method of claim 1, wherein the nickel-base superalloy has a
composition, in weight percent, of from about 4 to about 20 percent
cobalt, from about 1 to about 10 percent chromium, from about 5 to
about 7 percent aluminum, from 0 to about 2 percent molybdenum,
from about 3 to about 8 percent tungsten, from about 4 to about 12
percent tantalum, from 0 to about 2 percent titanium, from 0 to
about 8 percent rhenium, from 0 to about 6 percent ruthenium, from
0 to about 1 percent niobium, from 0 to about 0.1 percent carbon,
from 0 to about 0.01 percent boron, from 0 to about 0.1 percent
yttrium, from 0 to about 0.15 percent hafnium, balance nickel and
incidental impurities.
7. The method of claim 1, wherein the nickel-base superalloy has a
composition, in weight percent, selected from the group consisting
of
a composition of about 7.5 percent cobalt, about 7 percent
chromium, about 6.2 percent aluminum, about 6.5 percent tantalum,
about 5 percent tungsten, about 1.5 percent molybdenum, about 3
percent rhenium, about 0.05 percent carbon, about 0.004 percent
boron, about 0.15 percent hafnium, up to about 0.01 percent
yttrium, balance nickel and incidental impurities;
a composition of about 12.5 percent cobalt, about 4.2 percent
chromium, about 1.4 percent molybdenum, about 5.75 percent
tungsten, about 5.4 percent rhenium, about 7.2 percent tantalum,
about 5.75 percent aluminum, about 0.15 percent hafnium, about 0.05
percent carbon, about 0.004 percent boron, about 0.01 percent
yttrium, balance nickel and incidental impurities;
a composition of about 9.6 percent cobalt, about 6.6 percent
chromium, about 0.60 percent molybdenum, about 6.4 percent
tungsten, about 3.0 percent rhenium, about 6.5 percent tantalum,
about 5.6 percent aluminum, about 1.0 percent titanium, about 0.10
percent hafnium, balance nickel and incidental impurities;
a composition of about 7.00 percent cobalt, about 2.65 percent
chromium, about 0.60 percent molybdenum, about 6.40 percent
tungsten, about 5.50 percent rhenium, about 7.5 percent tantalum,
about 5.80 percent aluminum, about 0.80 percent titanium, about
0.06 percent hafnium, about 0.4 percent niobium, balance nickel and
incidental impurities;
a composition of about 5.00 percent cobalt, about 10.0 percent
chromium, about 4.00 percent tungsten, about 12.0 percent tantalum,
about 5.00 percent aluminum, about 1.5 percent titanium, balance
nickel and incidental impurities;
a composition of about 10.00 percent cobalt, about 5.00 percent
chromium, about 2.00 percent molybdenum, about 6.00 percent
tungsten, about 3.00 percent rhenium, about 8.70 percent tantalum,
about 5.60 percent aluminum, about 0.10 percent hafnium, balance
nickel and incidental impurities;
a composition of from about 0.4 to about 6.5 percent ruthenium,
from about 4.5 to about 5.75 percent rhenium, from about 5.8 to
about 10.7 percent tantalum, from about 4.25 to about 17.0 percent
cobalt, from 0 to about 0.05 percent hafnium, from 0 to about 0.06
percent carbon, from 0 to about 0.01 percent boron, from 0 to about
0.02 percent yttrium, from about 0.9 to about 2.0 percent
molybdenum, from about 1.25 to about 6.0 percent chromium, from 0
to about 1.0 percent niobium, from about 5.0 to about 6.6 percent
aluminum, from 0 to about 1.0 percent titanium, from about 3.0 to
about 7.5 percent tungsten, and wherein the sum of molybdenum plus
chromium plus niobium is from about 2.15 to about 9.0 percent, and
wherein the sum of aluminum plus titanium plus tungsten is from
about 8.0 to about 15.1 percent, balance nickel and incidental
impurities.
8. The method of claim 1, wherein the modified version of said
nickel-base superalloy has a composition, in weight percent, of
from about 0.2 to about 2.0 percent by weight hafnium, about 7.5
percent cobalt, about 7 percent chromium, about 6.2 percent
aluminum, about 6.5 percent tantalum, about 5 percent tungsten,
about 1.5 percent molybdenum, about 3 percent rhenium, about 0.05
percent carbon, about 0.004 percent boron, up to about 0.01 percent
yttrium, balance nickel and incidental impurities.
9. The method of claim 1, wherein the modified version of said
nickel-base superalloy has a composition, in weight percent, of
from about 0.1 to about 0.5 percent by weight zirconium, about 7.5
percent cobalt, about 7 percent chromium, about 6.2 percent
aluminum, about 6.5 percent tantalum, about 5 percent tungsten,
about 1.5 percent molybdenum, about 3 percent rhenium, about 0.05
percent carbon, about 0.004 percent boron, about 0.15 percent
hafnium, up to about 0.01 percent yttrium, balance nickel and
incidental impurities.
10. The method of claim 1, wherein the modified version of said
nickel-base superalloy has a composition, in weight percent, of
from about 0.2 to about 2.0 percent by weight hafnium, from about
0.1 to about 0.5 percent by weight zirconium, about 7.5 percent
cobalt, about 7 percent chromium, about 6.2 percent aluminum, about
6.5 percent tantalum, about 5 percent tungsten, about 1.5 percent
molybdenum, about 3 percent rhenium, about 0.05 percent carbon,
about 0.004 percent boron, up to about 0.01 percent yttrium,
balance nickel and incidental impurities.
11. The mood of claim 1, wherein the modified version of said
nickel-base superalloy has a composition, in weight percent, of
from about 0.2 to about 2.0 percent by weight hafnium, about 12.5
percent cobalt, about 4.2 percent chromium, about 1.4 percent
molybdenum, about 5.75 percent tungsten, about 5.4 percent rhenium,
about 7.2 percent tantalum, about 5.75 percent aluminum, about 0.05
percent carbon, about 0.004 percent boron, about 0.01 percent
yttrium, balance nickel and incidental impurities.
12. The method of claim 1, wherein the modified version of said
nickel-base superalloy has a composition, in weight percent, of
from about 0.1 to about 0.5 percent by weight zirconium, about 12.5
percent cobalt, about 4.2 percent chromium, about 1.4 percent
molybdenum, about 5.75 percent tungsten, about 5.4 percent rhenium,
about 7.2 percent tantalum, about 5.75 percent aluminum, about 0.15
percent hafnium, about 0.05 percent carbon, about 0.004 percent
boron, about 0.01 percent yttrium, balance nickel and incidental
impurities.
13. The method of claim 1, wherein the modified version of said
nickel-base superalloy has a composition, in weight percent, of
from about 0.2 to about 2.0 percent by weight hafnium, from about
0.1 to about 0.5 percent by weight zirconium, about 12.5 percent
cobalt, about 4.2 percent chromium, about 1.4 percent molybdenum,
about 5.75 percent tungsten, about 5.4 percent rhenium, about 7.2
percent tantalum, about 5.75 percent aluminum, about 0.05 percent
carbon, about 0.004 percent boron, about 0.01 percent yttrium,
balance nickel and incidental impurities.
14. The method of claim 1, wherein the step of processing includes
the step of
directionally solidifying the modified version of said nickel-base
superalloy composition.
15. The method of claim 1, wherein the protective layer is a
diffusion aluminide.
16. The method of claim 1, wherein the protective layer is a
NiCoCrA1Y(X) overlay, wherein X is selected from the group
consisting of hafnium, silicon, and tantalum.
17. The method of claim 1, wherein the substrate has a shape
selected from the group consisting of a turbine blade and a turbine
vane.
18. A method for preparing a single-crystal superalloy article
having a protective layer thereon, comprising the steps of
selecting a nickel-base superalloy composition;
preparing a modified version of said nickel-base superalloy
composition, wherein the modified version of said nickel-base
superalloy composition has an excess of a protective-layer
modifying element over that of the nickel-base superalloy
composition, the protective-layer modifying element being selected
from the group consisting of
from about 0.2 to about 2.0 percent by weight hafnium, and
from about 0.1 to about 0.5 percent by weight zirconium, and
combinations thereof;
processing the modified version of said nickel-base superalloy
composition into a substantially single-crystal substrate having
the shape of an article selected from the group consisting of a
turbine blade and a turbine vane;
applying a protective layer to a surface of the substrate, the
as-applied protective layer having a lower concentration of the
protective-layer modifying element than the substrate;
interdiffusing the protective layer modifying element from the
substrate into the applied protective layer; and
depositing a ceramic layer overlying the protective layer.
Description
BACKGROUND OF THE INVENTION
This invention relates to protective layers on nickel-base
superalloy articles, and, more particularly, to the fabrication of
such articles where the protective layer has a high content of
hafnium and/or zirconium.
In an aircraft gas turbine (et) engine, air is drawn into the front
of the engine, compressed by a shaft-mounted compressor, and mixed
with fuel. The mixture is combusted, and the resulting hot exhaust
gases are passed through a turbine mounted on the same shaft. The
flow of gas turns the turbine, which turns the shaft and provides
power to the compressor. The hot exhaust gases flow from the back
of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation
of the jet engine. There is thus an incentive to raise the exhaust
gas temperature. However, the maximum temperature of the exhaust
gases is normally limited by the materials used to fabricate the
turbine vanes and turbine blades of the turbine. In current
engines, the turbine vanes and blades are made of nickel-based
superalloys and can operate at temperatures of up to
1900-2100.degree. F.
Many approaches have been used to increase the operating
temperature limit and operating lives of the turbine blades and
vanes. The compositions and processing of the materials themselves
have been improved. The articles may be formed as oriented single
crystals to take advantage of superior properties observed in
certain crystallographic directions. Physical cooling techniques
are used. In one widely used approach, internal cooling channels
are provided within the components, and cool air is forced through
the channels during engine operation.
In another approach, a protective layer or a ceramic/metal thermal
barrier coating (TBC) system is applied to the turbine blade or
turbine vane component, which acts as a substrate. The protective
layer, with no overlying ceramic layer, is useful in
intermediate-temperature applications. The currently known
protective layers include diffusion aluminides and NiCoCrA1Y(X)
overlays, where X is typically hafnium, silicon, and/or
tantalum.
A ceramic thermal barrier coating layer may be applied overlying
the protective layer, to form a thermal barrier coating system. The
thermal barrier coating system is useful in higher-temperature
applications. The ceramic thermal barrier coating insulates the
component from the exhaust gas, permitting the exhaust gas to be
hotter than would otherwise be possible with the particular
material and fabrication process of the substrate.
Although superalloys coated with such protective layers and
ceramic/metal thermal barrier coating systems do provide
substantially improved performance over uncoated materials, there
remains an opportunity for improvement in elevated temperature
performance and environmental resistance. It has recently been
discovered that incorporating hafnium, silicon, yttrium, and/or
zirconium in the protective environmental coating improves its
environmental resistance and adherence to the substrate. However,
available techniques for applying protective layers with additions
of hafnium, silicon, yttrium, and/or zirconium have not proved to
be sufficiently reproducible for adoption in commercial fabrication
operations. There is a need for an improved approach to preparing
substrates having protective layers containing hafnium and/or
zirconium. The present invention fulfills this need, and further
provides related advantages.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method for preparing a superalloy
article with a protective layer thereon, wherein the protective
layer has an elevated hafnium and/or zirconium content, and the
articles made thereby. The approach retains excellent properties of
the underlying substrate, while achieving the benefits of these
protective-layer modifying elements. The approach is reliable and
reproducible, and therefore suited for production operations.
In accordance with the invention, a method for preparing a
superalloy article having a protective layer thereon comprises the
steps of selecting a nominal nickel-base superalloy composition,
and preparing a modified nominal nickel-base superalloy
composition. The modified nominal nickel-base superalloy
composition has an excess of a protective-layer modifying element
over that of the nominal nickel-base superalloy composition, where
the protective-layer modifying element is hafnium or zirconium. The
protective-layer modifying element is preferably present in the
modified nominal nickel-base superalloy composition in an amount of
from about 0.2 to about 2.0 percent by weight, preferably about 1.0
percent by weight, for the case of hafnium, and/or in an amount of
from about 0.1 to about 0.5 percent by weight, preferably about
0.25 percent by weight, for the case of zirconium, and combinations
thereof. The method further includes processing the modified
nominal nickel-base superalloy composition into a single crystal
substrate having the shape of the article, and applying a
protective layer to a surface of the substrate. The as-applied
protective layer has a lower concentration of the protective-layer
modifying element than the substrate. The protective layer
modifying element is diffused from the substrate into the applied
protective layer, providing an enhanced level of the protective
layer modifying element in the protective layer.
The protective layer may be a diffusion aluminide or a NiCoCrA1Y(X)
overlay, or other type of layer that benefits from the presence of
increased amounts of hafnium and/or zirconium. Optionally, a
ceramic layer may be deposited overlying the enhanced protective
layer to form a thermal barrier coating system.
In prior fabrication approaches, the hafnium, silicon, yttrium, or
zirconium have been added to the material deposited on the surface
of the substrate that forms the protective layer. These fabrication
techniques have proved to be insufficiently reproducible and
reliable. In the present approach, by contrast, hafnium and/or
zirconium in an elevated amount is added to the substrate alloy,
and then diffused outwardly into the protective layer. This permits
the protective layer which need not have hafnium, silicon, yttrium,
or zirconium as it is deposited, to be deposited by more
conventional techniques that are reliable and reproducible. There
is concern that the presence of the protective-layer modifying
element in the substrate in excessive amounts may adversely affect
its mechanical properties. For this reason, only hafnium and/or
zirconium, and not silicon and/or yttrium, are added in extra
amounts to the substrate alloy.
Further, the hafnium and/or zirconium are added to the substrate in
a specific narrow compositional range such that the benefits of
their increased levels in the protective layer are realized without
adverse effects on the mechanical properties of the substrate. The
protective-layer modifying element is preferably present in the
modified nominal nickel-base superalloy composition in an amount of
from about 0.2 to about 2.0 percent by weight, preferably about 1.0
percent by weight, for the case of hafnium; and/or in an amount of
from about 0.1 to about 0.5 percent by weight, preferably about
0.25 percent by weight, for the case of zirconium. Combinations of
hafnium and zirconium within these compositional ranges are
operable.
Other features and advantages of the present invention will be
apparent from the following more detailed description of the
preferred embodiment, taken in conjunction with the accompanying
drawings, which illustrate, by way of example, the principles of
the invention. The scope of the invention is not, however, limited
to this preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a turbine blade;
FIGS. 2A-B are schematic enlarged sectional views of the article of
FIG. 1, taken on line 2--2, illustrating coating systems on the
surface of the article, wherein FIG. 2A illustrates an
environmental coating and FIG. 2B illustrates a thermal barrier
coating system; and
FIG. 3 is a block diagram of a preferred embodiment of the approach
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 depicts a component article of a gas turbine engine such as
a turbine blade or turbine vane, and in this illustration a turbine
blade 20. The turbine blade 20 includes an airfoil 22 against which
the flow of hot exhaust gas is directed. (The turbine vane has a
similar appearance in respect to the pertinent airfoil portion.) At
least the airfoil 22, and preferably the entire turbine blade 20,
is substantially single crystal. That is there are substantially no
grain boundaries in the single crystal portion, and the
crystallographic orientation is the same throughout. The term
"substantially single crystal" means that virtually the entire
article is a single crystal, although there may be some incidental
small regions having other crystalline orientations present. Even a
substantially single crystal article typically has a number of
low-angle grain boundaries present, and these are permitted within
the scope of the term "substantially single crystal".
The article must be substantially a single crystal (i.e., single
grain). It may not be a polycrystal, either a random polycrystal or
an oriented polycrystal such as produced by directional
solidification. In the polycrystalline alloys, it has been
conventional to add higher levels of elements that are known to
strengthen grain boundaries, such as carbon, boron, hafnium, and
zirconium. Zirconium and hafnium are chemically reactive, modify
the morphologies of precipitate phases, and may adversely affect
the heat treatment of the alloys. Because these elements are not
needed to strength high-angle grain boundaries, which are not
present in substantially single-crystal articles, it has therefore
been the prior practice to omit them from single-crystal articles
except in very minor amounts to strengthen the low-angle grain
boundaries that may be present.
The turbine blade 20 is mounted to a turbine disk (not shown) by a
dovetail 24 which extends downwardly from the airfoil 22 and
engages a slot on the turbine disk. A platform 26 extends
longitudinally outwardly from the area where the airfoil 22 is
joined to the dovetail 24. In some articles, a number of cooling
channels extend through the interior of the airfoil 22, ending in
openings 28 in the surface of the airfoil 22. A flow of cooling air
is directed through the cooling channels, to reduce the temperature
of the airfoil 22.
FIGS. 2A and 2B illustrate coating systems 30a and 30b deposited
upon the turbine blade 20, which thereby serves as a substrate 32.
The coating systems 30a and 30b include a protective layer 34
overlying and contacting a surface 36 of the turbine blade 20. In
the coating system 30a of FIG. 2A, the protective layer 34 is
sometimes termed an "environmental coating", and in the coating
system 30b of FIG. 2B, the protective layer 34 is sometimes termed
a "bond coat". In each case, the protective layer 34 is preferably
from about 0.0005 to about 0.004 inches in thickness, but lesser or
greater thicknesses are operable although less desirable. In each
of the coating systems 30a and 30b, a topmost surface 38 of the
protective layer 34 oxidizes during fabrication and/or during
service to form a thin aluminum oxide layer 39. The term
"protective layer" as used herein encompasses both environmental
coatings (having no overlying ceramic thermal barrier coating, as
in FIG. 2A) and bond coats (having an overlying ceramic thermal
barrier coating 40, as in FIG. 2B).
The substrate 32 is formed of a modified nominal nickel-base
superalloy composition. As used herein, "nickel-base" means that
the composition has more nickel present than any other element. The
preferred nominal nickel-base superalloys have a composition, in
weight percent, of from about 4 to about 20 percent cobalt, from
about 1 to about 10 percent chromium, from about 5 to about 7
percent aluminum, from 0 to about 2 percent molybdenum, from about
3 to about 8 percent tungsten, from about 4 to about 12 percent
tantalum, from 0 to about 2 percent titanium, from 0 to about 8
percent rhenium, from 0 to about 6 percent ruthenium, from 0 to
about 1 percent niobium, from 0 to about 0.1 percent carbon, from 0
to about 0.01 percent boron, from 0 to about 0.1 percent yttrium,
from 0 to about 0.15 percent hafnium, balance nickel and incidental
impurities.
A most preferred nominal nickel-base superalloy composition is
Rene' N5, which has a nominal composition in weight percent of
about 7.5 percent cobalt, about 7 percent chromium, about 6.2
percent aluminum, about 6.5 percent tantalum, about 5 percent
tungsten, about 1.5 percent molybdenum, about 3 percent rhenium,
about 0.05 percent carbon, about 0.004 percent boron, about 0.15
percent hafnium, up to about 0.01 percent yttrium, balance nickel
and incidental impurities. Other operable superalloys include, for
example, Rene' N6, which has a nominal composition in weight
percent of about 12.5 percent cobalt, about 4.2 percent chromium,
about 1.4 percent molybdenum, about 5.75 percent tungsten, about
5.4 percent rhenium, about 7.2 percent tantalum, about 5.75 percent
aluminum, about 0.15 percent hafnium, about 0.05 percent carbon,
about 0.004 percent boron, about 0.01 percent yttrium, balance
nickel and incidental impurities; CMSX-4, which has a nominal
composition in weight percent of about 9.60 percent cobalt, about
6.6 percent chromium, about 0.60 percent molybdenum, about 6.4
percent tungsten, about 3.0 percent rhenium, about 6.5 percent
tantalum, about 5.6 percent aluminum, about 1.0 percent titanium,
about 0.10 percent hafnium, balance nickel and incidental
impurities; CMSX-10, which has a nominal composition in weight
percent of about 7.00 percent cobalt, about 2.65 percent chromium,
about 0.60 percent molybdenum, about 6.40 percent tungsten, about
5.50 percent rhenium, about 7.5 percent tantalum, about 5.80
percent aluminum, about 0.80 percent titanium, about 0.06 percent
hafnium, about 0.4 percent niobium, balance nickel and incidental
impurities; PWA1480, which has a nominal composition in weight
percent of about 5.00 percent cobalt, about 10.0 percent chromium,
about 4.00 percent tungsten, about 12.0 percent tantalum, about
5.00 percent aluminum, about 1.5 percent titanium, balance nickel
and incidental impurities; PWA1484, which has a nominal composition
in weight percent of about 10.00 percent cobalt, about 5.00 percent
chromium, about 2.00 percent molybdenum, about 6.00 percent
tungsten, about 3.00 percent rhenium, about 8.70 percent tantalum,
about 5.60 percent aluminum, about 0.10 percent hafnium, balance
nickel and incidental impurities; and MX-4, which has a nominal
composition as set forth in U.S. Pat. No. 5,482,789, in weight
percent, of from about 0.4 to about 6.5 percent ruthenium, from
about 4.5 to about 5.75 percent rhenium, from about 5.8 to about
10.7 percent tantalum, from about 4.25 to about 17.0 percent
cobalt, from 0 to about 0.05 percent hafnium, from 0 to about 0.06
percent carbon, from 0 to about 0.01 percent boron, from 0 to about
0.02 percent yttrium, from about 0.9 to about 2.0 percent
molybdenum, from about 1.25 to about 6.0 percent chromium, from 0
to about 1.0 percent niobium, from about 5.0 to about 6.6 percent
aluminum, from 0 to about 1.0 percent titanium, from about 3.0 to
about 7.5 percent tungsten, and wherein the sum of molybdenum plus
chromium plus niobium is from about 2.15 to about 9.0 percent, and
wherein the sum of aluminum plus titanium plus tungsten is from
about 8.0 to about 15.1 percent, balance nickel and incidental
impurities. The use of the present invention is not limited to
these alloys, and has broader applicability.
The nature of the modifications to the nominal nickel-base
superalloys will be discussed more fully subsequently. However, for
all of these compositions set forth in the preceding two
paragraphs, where the protective-layer modifying element is
hafnium, in the modified nominal nickel-base superalloy composition
the hafnium content of the nominal nickel-base superalloy is
replaced by the hafnium content in its specified range of from
about 0.2 to about 2.0 percent by weight. Where the
protective-layer modifying element is zirconium, in the modified
nominal nickel-base superalloy composition the zirconium content is
as stated within its specified range of from about 0.1 to about 0.5
percent by weight and the hafnium content is as indicated for the
nominal nickel-base superalloy. Where the protective-layer
modifying element is a combination of hafnium and zirconium, in the
modified nominal nickel-base superalloy composition the hafnium
content of the nominal nickel-base superalloy is replaced by the
hafnium content in its specified range from about 0.2 to about 2.0
percent by weight and the zirconium content is as stated within its
specified range of from about 0.1 to about 0.5 percent by
weight.
The protective layer 34 may be of any operable type, but preferably
is either a diffusion aluminide or a NiCoCrA1Y(X) overlay. A
diffusion aluminide is formed by depositing one or more sublayers
overlying the surface 36, and then interdiffusing the deposited
sublayers. For example, a sublayer containing platinum is first
deposited upon the surface 36, and then a sublayer containing
aluminum is deposited over the platinum sublayer at a temperature
sufficient that the platinum and aluminum sublayers interdiffuse to
form a platinum-aluminum coating layer. In a preferred embodiment,
the platinum is present in an average amount of from about 20 to
about 30 weight percent, preferably about 25 to about 28 weight
percent, of the protective layer 34, and the aluminum is present in
an average amount of from about 14 to about 25 weight percent,
preferably from about 18 to about 22 weight percent, of the
protective layer 34. In the NiCoCrA1Y(X) overlay coating, a layer
having the composition NiCoCrA1Y(X), where X may be hafnium,
silicon, and/or tantalum, is deposited on the surface 36. The
NiCoCrA1Y(X) overlay coating may have a composition in weight
percent, for example, of 20 percent cobalt, 18 percent chromium, 12
percent aluminum, 0.3 percent yttrium, 0.5 percent hafnium, 0.5
percent silicon, balance nickel.
The ceramic thermal barrier coating layer 40, where present, is
preferably from about 0.004 inches to about 0.030 inches thick,
most preferably from about 0.005 to about 0.015 inches thick.
(FIGS. 2A and 2B are not drawn to scale.) The ceramic thermal
barrier coating layer 40 is operable in thicknesses outside this
range, but is less desirable. Lesser thicknesses of the ceramic
thermal barrier coating layer 40 tend to give insufficient
insulation to the substrate 32. Greater thicknesses of the ceramic
thermal barrier coating layer 40 tend to add unnecessary weight to
the article. The ceramic thermal barrier coating layer 40 is
preferably yttria-(partially) stabilized zirconia, which is a
zirconium oxide-base ceramic material containing from about 4 to
about 8 weight percent of yttrium oxide. Other operable stabilizing
oxides and ceramic base materials may be used as well.
It is desirable to modify the composition of the protective layer
34 with controlled additions of one or more of the elements
hafnium, zirconium, yttrium, and/or silicon. The presence of these
elements in the protective layer 34 reduces the incidence of
spallation failure within the protective layer 34 and/or the
aluminum oxide layer 39, thereby prolonging the service life of the
coating system 30.
However, experience has shown that it is difficult to incorporate
these elements hafnium, zirconium, yttrium, and/or silicon, in the
amounts required to be effective, into the protective layer 34
using conventional processing techniques for the diffusion
aluminide or overlay coatings.
The present invention provides an alternative fabrication
processing that permits a protective-layer modifying element to be
incorporated into the protective coating in the amounts required to
be effective in improving the properties of the protective layer
34. FIG. 3 depicts a preferred approach to practicing the present
invention.
A nominal substrate composition is selected, numeral 50. This
composition is selected as appropriate to the application, such as
a nickel-base alloy appropriate to the preparation of a single
crystal turbine blade or turbine vane. Examples of such nominal
substrate compositions include Rene' N5. Rene' N6, CMSX-4, CMSX-10,
PWA 1480, PWA 1484, and MX-4, whose compositions have been set
forth previously.
A modified nominal nickel-base superalloy substrate composition is
prepared, numeral 52. The modified nominal nickel-base superalloy
substrate composition is the nominal nickel-base superalloy
composition selected in step 50, modified by the addition of an
excess of a protective-layer modifying element, above that which
would otherwise be present in the nominal nickel-base superalloy
substrate composition. The protective-layer modifying element is
either hafnium or zirconium.
The hafnium or zirconium must be present in the modified nominal
nickel-base superalloy composition in a concentration not less than
(i.e., equal to or greater than) the concentration that is to be
present in the final protective layer 34. However, the hafnium or
zirconium may not be present in the modified nominal nickel-base
superalloy composition in an amount that would have a substantial
adverse effect on the mechanical and/or physical properties of the
modified nominal nickel-base superalloy composition in its service
application. For these reasons, only hafnium and zirconium have
been determined to be candidates for the protective-layer modifying
element. Other elements, such as silicon and yttrium, which may
potentially improve the properties of the protective-layer must be
added to the substrate composition in too great a concentration to
be acceptable. For example, the amount of silicon necessary to
impart beneficial effects to the properties of the protective layer
would require its concentration to be so large in the substrate
composition that it would adversely affect the properties of the
substrate material through increased long-term microstructural
instability. The amount of yttrium necessary to impart beneficial
effects to the properties of the protective layer would require its
concentration to be so large in the substrate composition that it
would cause excessive incipient melting during solution heat treat.
Silicon and yttrium additions to the nominal nickel-base superalloy
composition therefore do not come within the scope of the present
invention.
The protective-layer modifying element is preferably present in the
modified nominal nickel-base superalloy composition in an amount of
from about 0.2 to about 2.0 percent by weight, preferably about 1.0
percent by weight, for the case of hafnium; and/or in an amount of
from about 0.1 to about 0.5 percent by weight, preferably about
0.25 percent by weight, for the case of zirconium. If the amount of
the addition is less than the indicated minimum in each case, there
is an insubstantial advantageous effect on the properties of the
protective-layer. If the amount of the addition is greater than the
indicated maximum in each case, the mechanical and/or physical
properties of the substrate are adversely affected. Other
properties such as castability and heat treatability are also
adversely affected if the amount of the addition is too great.
Thus, for example, a preferred modified Rene' N5 composition, in
weight percent, is 7.5 percent cobalt, 7 percent chromium, 6.2
percent aluminum, 6.5 percent tantalum, 5 percent tungsten, 1.5
percent molybdenum, 3 percent rhenium, 0.05 percent carbon. 0.004
percent boron, up to 0.01 percent yttrium, about 1.0 percent
hafnium, balance nickel and incidental impurities. In a variation
of this preferred modified superalloy composition, the alloy is
prepared with its conventional nominal hafnium content of about
0.15 weight percent, and about 0.25 percent zirconium is added.
This amount of hafnium and/or zirconium in the superalloy
composition does not substantially adversely affect the properties
of the superalloy in service.
The modified nominal nickel-base substrate composition is processed
into the desired shape of the substrate 32, numeral 54. The
processing is desirably accomplished by casting, preferably by a
casting process which produces a single crystal structure in the
final article, such as the turbine blade 20.
The protective layer 34 is applied overlying the surface 36 of the
substrate 32, numeral 56. The protective layer 34 is applied by any
approach which is operable to produce the desired protective layer,
and the selected approach depends upon the type of protective layer
chosen. For the diffusion aluminide such as a platinum aluminide,
the platinum layer may be electrodeposited and an overlying
aluminum layer deposited by chemical vapor deposition or pack
cementation. For the NiCoCrA1Y(X) overlay coating, the coating may
be deposited by low-pressure plasma spray (LPPS), air plasma spray
(APS), high velocity oxyflame deposition (HVOF), or electron beam
physical vapor deposition (EB-PVD). The protective layer 34 does
not include so great an amount of the protective-layer modifying
element that the conventional application procedure may not be
used, and in most cases none of the protective-layer modifying
element is present in the as-applied protective layer, except as
may diffuse into the protective layer from the substrate.
The substrate 32 and the protective layer 34 are heated so that the
protective-layer modifying element interdiffuses from the substrate
32 into the protective layer 34, numeral 58. The heating step 58
may be performed in part simultaneously with the application step
56, because the application step 56 is typically performed at
elevated temperature. The heating step 58, with the resultant
interdiffusion, may continue after the application step 56 is
complete. The heating step 58 may also be performed simultaneously
with subsequent processing steps that require heating to elevated
temperature, and even during elevated-temperature service of the
article. Preferably, the interdiffusion step 58 involves at least a
heating to a temperature of from about 1900.degree. F. to about
2000.degree. F., for a time of at least about 2 hours.
The ceramic layer 40 is deposited, numeral 60, in those cases where
the system is to serve as a thermal barrier coating system. The
ceramic coating 40 may be deposited by any operable technique, with
electron beam physical vapor deposition (EB-PVD) being preferred
for the preferred yttria-stabilized zirconia coating. The EB-PVD
processing may be preceded and/or followed by high-temperature
processes that may affect the distribution of elements in the bond
coat. The EB-PVD process itself is typically conducted at elevated
temperatures.
During the interdiffusion processing, the protective-layer
modifying element diffuses from the substrate 32 outwardly into the
protective layer 34. These elements alloy with the as-deposited
protective layer 34 to form the modified composition, which is more
highly resistant to failure in service than the unmodified
composition.
Test specimens were prepared of the Rene N5 nominal composition as
set forth above, and six compositions having the Rene N5 nominal
composition plus 0.64 weight percent hafnium, 1.06 weight percent
hafnium. 1.33 weight percent hafnium, 0.2 weight percent zirconium,
0.5 weight percent zirconium, or 0.75 weight percent zirconium. All
of these alloys were easily made into single crystal slabs without
any reaction with the mold materials, an important consideration
for production operations. The compositions were heat treated at a
temperature of 2200-2400.degree. F. for up to 16 hours, and
specimens were prepared of some of the composition for the
evaluation of thermal barrier coating performance. (No testing was
performed for the 0.5 weight percent zirconium and 0.75 weight
percent zirconium compositions.)
To evaluate thermal barrier coating performance, multiple 1 inch
diameter by 1/8+L inch thick disk specimens were first coated with
a platinum aluminide bond coat and then a ceramic top coat as
discussed earlier. The test procedure was a furnace cycling test in
which the specimen was heated from room temperature to 2125.degree.
F., maintained at 2125.degree. F. for 50 minutes, and cooled to
room temperature, each total cycle lasting 60 minutes. Failure was
defined as spalling away of 20 percent of the coating area. The
results are as follows, in the average number of cycles to failure
taken over the indicated number of specimens:
Specimen Avg. Cycles to No. of Identification Failure Specimens
Rene N5 228 40 Rene N5 + 0.64 percent hafnium 340 6 Rene N5 + 1.06
percent hafnium 397 6 Rene N5 + 1.33 percent hafnium 480 6 Rene N5
+ 0.2 percent zirconium 330 6
The approach of the invention yields a substantial improvement in
the thermal barrier coating performance as measured by the furnace
cycling test.
Specimens were also prepared of some of the composition for the
evaluation of mechanical properties in stress rupture testing. (No
testing was performed for the 0.5 weight percent zirconium and 0.75
weight percent zirconium compositions.) In a first test protocol,
specimens were tested at 1800.degree. F. and 30,000 pounds per
square inch stress. In a second test protocol, specimens were
tested at 2000.degree. F. and 16,000 pounds per square inch stress.
The number of hours to failure for each test protocol is set forth
in the following table, with each data entry being the average of
four tests.
Specimen First protocol, Second protocol, Identification hours
hours Rene N5 300 400 Rene N5 + 0.64 percent hafnium 342 771 Rene
N5 + 1.06 percent hafnium 329 454 Rene N5 + 1.33 percent hafnium
294 236 Rene N5 + 0.2 percent zirconium 348 504
From this data and other information, the limitations on the
hafnium and zirconium contents as set forth above were
established.
Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various
modifications and enhancements may be made without departing from
the spirit and scope of the invention. Accordingly, the invention
is not to be limited except as by the appended claims.
* * * * *